Aircraft are generically divided into two major classes, fixed wing and rotating airfoil. The former are typically thought of as "airplanes", and derive their lift from the forward motion of the machine, which causes air to pass over an airfoil. Rotary winged aircraft, commonly called "helicopters", have a prime mover attached to an airfoil, which rotates. The primary drawback to conventional aircraft is that they must have rather large areas to take off and land. The principal drawback to the helicopter is that it is extremely expensive to manufacture, has little inherent control stability, and is capable of very low forward speeds, compared to an aircraft having a prime mover of the same power. The most successful short take-off and landing aircraft is the helicopter, whose rotary system produces the necessary lift. To reduce high speed vibration and drag of the rotor, stub wings are sometimes added to produce lift at forward speeds, thereby reducing load on the rotor. The results, in various configurations, are called the convertaplane. In hovering, however, the down wash of the rotor produces large loads on the wings, which compromises hovering performance.
To avoid these difficulties, several alternatives have been tried. These range from tilting the entire aircraft ninety degrees (90.degree.) after vertical takeoff, as was done with the "Bell Pogo" aircraft, which used counter-rotating propellers on the nose of the aircraft, to tilting the power plant or wings, in combination. The same effect can be obtained by running jet engines in a horizontal position and deflecting the jet blast downward, to effect vertical thrust for take-off. When sufficient altitude has been gained, the deflection vanes are retracted and the aircraft moves in level flight. The most spectacular example of this class is the British Harrier military aircraft, which can rise vertically from an area little greater than its own overall dimensions, then achieve supersonic speed in level flight. The process can be reversed for vertical landing.
All direct-lift machines known to prior art have certain problems in common. The first is the detrimental effect of the high energy slip stream or jet striking the ground. Loose material thrown about constitutes a hazard, both to the machine and to personnel in the vicinity. Also, while the entire lifting force depends upon the engines while hovering at low speeds, power failure could prove catastrophic. The greatest possible reliability in adequate emergency back-up systems is necessary in order to assure the safety of the machine and its occupants.
Because there is little or no flow over the aerodynamic control surfaces during hovering flight, entirely different types of thrust vector controls have been utilized by V/STOL's during hovering or slow forward flight. As of the early 1980's, none of these systems are entirely satisfactory.
U.S. Pat. No. 3,335,976 shows an aircraft, like the Ryan XV-5A, incorporating lift fans in the large relatively thick main lift wings and using clamshell shutters to close off the lift wing during forward flight. This design requires thick main wings, which create unacceptable drag during forward flight.
U.S. Pat. No. 4,194,708, teaches the use of a deflectable canard/elevator placed close to the nose tip of the aircraft, with wings mounted low and well aft on the fuselage. U.S. Pat. No. 3,618,875 teaches a V/STOL aircraft having tandem wings containing lift fans, wherein the wings provide only drag during forward flight.
The prior art teaches that, in order to maintain sufficient vertical thrust to operate a VTOL, in-the-wing fans have always necessitated the use of aircraft wings of relatively large area. This has led to the development of many designs such as that shown in U.S. Pat. No. 3,388,878, wherein fuselage mounted lift fans and gas generators lead to very complex retractable lift fan installations. It has been the object of much of the prior art of wing-mounted lift fans to minimize wing area, wing weight and general complexity, because the wings containing the lift fan were not used to provide aerodynamic lift during forward flight. For these reasons full potential of lift fans concept has not been fully realized in V/STOL aircraft designs proposed to date.
U.S. Pat. No. 3,614,030 teaches a disk-like aircraft body forming an axis of revolution, wherein individually controlled rotary members movable about the air flow axis control the direction of discharge of air from the ducts. U.S. Pat. No. 3,614,030, however, does not teach an aircraft using conventional aerodynamics lift, but, rather, a ground effect machine.
Generally speaking, these, and other prior art proposals have sought to produce V/STOL aircraft capable of obtaining the vertical flight characteristics of helicopters and the forward flight characteristics of fixed wing aircraft. Such a hybrid provides a potential solution to the air traffic problems of congestion, both in conventional airports and in land transportation to and from conventional airports. The much lower approach speeds of V/STOL aircraft can permit many more aircraft to safely occupy the air space for multiple take-offs and landings.
Previously proposed fixed wing V/STOL aircraft have encountered many problems in assuring safe and reliable operation. From a practical standpoint, prior fixed wing V/STOL aircraft have also had such a high direct operating cost that they are commercially unattractive.